Molding of polypropylene with enhanced reheat characteristics

ABSTRACT

Bottles, containers and other articles are formed from polypropylene compositions that include a reheating agent, such as antimony, carbon black, graphite, titanium, copper, manganese, iron, tungsten, graphite, infra-red absorbing dyes or other infra-red absorbing material. The reheating time for the polypropylene composition is shortened for injection stretch blow molding or thermoforming, and the polypropylene granule composition with reheating agent has an L* value of at least 80% of the L* value for a polypropylene granule control without added reheating agents as measured by the Gardner color test. The reheating agent may be incorporated into the polypropylene composition by in situ chemical reduction of a metal compound, such as antimony triglycolate, with a reducing agent, such as hypophosphorous acid. In addition, the polypropylene composition with reheating agent may be derived from a polypropylene masterbatch with high concentrations of reheating agent.

FIELD OF THE INVENTION

This invention relates to the manufacture of bottles, containers andother articles from polypropylene polymer compositions, in particular byinjection stretch blow molding and thermoforming techniques.

BACKGROUND OF THE INVENTION

Polyester compositions, such as polyethylene terephthalate or copolymersthereof (hereinafter collectively referred to as “PET”), are well knownpackaging materials. For example, in U.S. Pat. No. 4,340,721, a PETcomposition is used to manufacture beverage bottles and other containers(hereinafter referred to as “bottles”) by various molding methods.

In current practice PET bottles of the size and shape for most beverageapplications are usually made by an injection stretch blow moldingtechnique. Injection stretch blow molding has two main steps. First, thePET, in the form of granules, is melted in an injection molding machineand the melt injected into a cooled mold to form a precursor to thefinal bottle known as a “preform”. Commonly, the preform has a threadedneck with a shortened bottle body shape length about 8 to 20 cm and amaterial thickness between 3 mm and 6 mm. Second, the preform istransferred to a stretch blow molding machine where its externalsurfaces are reheated by infra-red (IR) lamps. Once the preform hasreached a desired temperature, it is stretched and blown to form thefinal bottle.

The time it takes to reheat the preform is the rate-limiting factor forthe overall process. The preform starts at ambient temperature and hasto be heated to above the glass transition temperature of the polyester(generally to about 110° C.) so that the preform becomes sufficientlyflexible to permit the stretch-blow step to work. In general, polyesterpolymers have a poor ability to absorb IR radiation. Hence, as well asextending the overall production time, the preform reheating step alsorequires a significant amount of energy. To address this problem,certain prior patents have taught that adding black materials and/ormetal particles to PET compositions can reduce the time and energyrequired for reheating. Hence, prior patents teach adding carbon black(U.S. Pat. No. 4,476,272), iron oxide (U.S. Pat. No. 4,250,078), andantimony and other metal particles (U.S. Pat. Nos. 5,419,936 and5,529,744) to reduce PET preform reheating time. Antimony metalparticles were indicated as preferred because such particlespreferentially absorb radiation at or near the infra-red wavelengthsemitted by the IR lamps in most stretch blow mold machines, e.g., 500 nmto 2000 nm. Furthermore, as described in U.S. Pat. Nos. 5,419,936 and5,529,744, antimony compounds are usually present in the polyestercomposition itself (as the catalyst for melt polymerization) and can beconverted to antimony metal particles, with the desired IR absorptioncharacteristics, by the addition of a reducing agent in the meltpolymerization stage of manufacture.

Although PET has found widespread application for beverage bottles, thecost of raw materials for making PET is much higher than for somenon-PET polymers. Therefore, the industry continually seeks to switchfrom PET to lower cost alternatives. Whilst seeking these alternatives,container manufacturers do not wish to invest substantial resources innew capital equipment to process a new polymer material, but wouldprefer to adapt their existing PET injection blow molding equipment foruse with the new material.

One possible alternative to PET for use in injection stretch blowmolding of beverage bottles is polypropylene. U.S. Pat. No. 6,258,313teaches that injection stretch blow molding of a polypropylene preformis possible if the preform is heated simultaneously both from theoutside and inside. Nevertheless, it has heretofore been more difficultto produce satisfactory beverage bottles from polypropylene by thismethod. First, polypropylene has a lower density and specific heat thanPET and hence exhibits a significantly narrower processing window.Second, polypropylene suffers from the same limitations as PET in termsof its poor ability to absorb IR radiation. Furthermore, polypropylenegenerally has greater opacity than PET, which detracts from itsaesthetic appearance. The industry thus continues to seek ways toimprove the IR absorption properties of polypropylene such that it canbe used to make beverage bottles on the same injection stretch blowmolding equipment as PET and/or used to make other thermoformedarticles.

SUMMARY OF THE INVENTION

In a first aspect, the invention is a method for injection stretch blowmolding a polypropylene bottle from a preform that comprises apolypropylene composition containing a reheating agent. The reheatingagent may be one or more metal particles of antimony, titanium, copper,manganese, iron and tungsten, where antimony is most preferred. Thereheating agent also may be particles of carbon black, graphite,infra-red absorbing dyes or other infra-red absorbing material.

The preform is reheated, usually by heating with one or more heatinglamps, to a desired reheat temperature. The time for reheating thepreform is shorter than the time for reheating a control preform ofequivalent dimensions formed from the polypropylene composition withouta reheating agent. The polymer granules used to make the preform have anL* value of at least about 80% of an L* value of the polymer granulesused to make the control preform. The L* values are measured by theGardner color test. For example, where the control polypropylenecomposition granules have an L* value of about 75, the granules used tomanufacture the preform according to the invention have an L* value ofabout 60 or above. The closer the L* value is to the L* value of thecontrol, the more the final bottle will resemble the color/aesthetics ofa bottle made from the control polypropylene.

Preferably, the reheating agent is incorporated into the polypropylenein the form of particles having particle sizes in the range of about 10nanometers (nm) to about 100 micrometers, and more preferably in therange of 10 nm to 10 microns. Preferably, the reheating agent particlesare incorporated into the polypropylene in an amount in the range ofabout 2 ppm to 1000 ppm, more preferably from 2 ppm to 350 ppm, mostpreferably from 2 ppm to 50 ppm. The reheating agent particles also maybe incorporated into the polypropylene composition in the form ofparticles having sizes in the range of 10 nm to 100 microns and in anamount in the range of 50 ppm to 25,000 ppm to form a polypropylenemasterbatch. The masterbatch then may be blended with otherpolypropylene compositions (possibly free of reheating agents orcontaining different reheating agents or different proportions of thesame reheating agents) to form a polypropylene composition with desiredproportion(s) of reheating agent(s).

In addition, the reheating agent may be generated within thepolypropylene composition by in situ chemical reduction of a metalcompound with a reducing agent.

Thus, the metal compound can contain one or more of antimony, titanium,copper, manganese, iron and tungsten, and the reducing agent can be oneor more organic phosphorous acids or inorganic phosphorous acids, ortannic, gallic, and pyrogallic acids, or hydrazine, or sulphites, or tinII salts, nickel hydroxide or any organic or inorganic compound with anelectrochemical potential sufficient to reduce the metal compounds tothe metallic state. Preferably, the metal compound is antimonytriglycolate and the reducing agent is hypophosphorous acid.

The preforms, polypropylene bottles and other polypropylene articlesmade from the polypropylene compositions with reheating agents are alsoclaimed. Examples of other polypropylene articles includethree-dimensional articles, such as cups or serving trays or foodcontainers, and two-dimensional articles, such as sheets. Depending uponthe desired aesthetics of the final articles, the L* values for thepolypropylene granules with reheating agents used to form these otherpolypropylene articles may be outside the preferred range for L* forcompositions used to form bottle preforms that are injection stretchblow molded into bottles.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in the following detaileddescription with reference to the following drawings:

FIGS. 1A to 1D are schematic diagrams of the typical steps in aninjection stretch blow molding method for bottle making;

FIG. 2 is a schematic diagram showing an apparatus for heating a polymerplaque with a single IR lamp, which apparatus may be used to determinethe through heating time of the polymer;

FIG. 3 is a graph of plaque temperature data over reheating time forplaque surface reheat experiments conducted with plaques formed withdifferent polypropylene compositions;

FIG. 4 is a graph of data showing the variation in the time required tothrough heat a polypropylene plaque to a target temperature (i.e., 80°C.) in relation to the amount of reheat agent in the polypropylenecomposition forming the plaque; and

FIG. 5 is a graph of data comparing reheat time (seconds to 80° C.)versus the amount of reheat agent added to the polypropylenecompositions forming the plaques; and

FIG. 6 is a graph of data comparing L* (for degree of color) versus theamount of reheating agent added to the polypropylene compositionsforming the plaques.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to polypropylene compositions that areuseful in the production of stretch blow molded containers, particularlybottles, with good color quality and transparency. Such compositions mayalso be used to make other polypropylene containers and polypropylenearticles with other two-step techniques, such as thermoforming. Thepolypropylene compositions contain as reheating agents certain metalparticles that intrinsically absorb radiation in the wavelength regionof 500 nm to 2000 nm, which are the typical wavelengths of the radiationfrom infra-red lamps used in IR heating in PET injection stretch blowmolding. The metal particles are present in sufficient amount to reducethe reheating time that would otherwise be required to reheat a preformof a polypropylene composition to the desired temperature duringinjection stretch blow molding or thermoforming. Such polypropylenecompositions with reheating agents still have acceptable color andclarity for the desired end use applications.

Referring first to FIGS. 1A to 1D, a known injection stretch blowmolding method involves certain steps. First, an injection moldedpreform 10 as shown in FIG. 1A has a threaded neck portion 12 and abottle body portion 14. The preform 10 is injection molded from either apolypropylene composition containing reheat agents or from a reheatagent concentrate masterbatch of granules that are mixed with polymergranules without reheat agents. Most commonly, each polypropylenegranule is from 2.5 to 4.0 mm long and 2.0 to 3.0 mm in diameter. Thepolypropylene composition or mixture of polypropylene granules is heatedto melt the composition/granules to form a flowable polymer melt that isintroduced by injection into the mold. The injection mold has a cavityand a mating ram to form the preform into the desired contoured shape.The preform 10 is removed from the mold, cooled and stored until it isready to be formed into a bottle.

As shown in FIG. 1B, the preform 10 is then installed over a fixture 18and held within a reheating or preheating cavity 20. Heating lamp 22,which may be one or a series of lamps, emits infra-red radiation thatheats the outer surface of the preform 10 as the preform 10 is rotatedon the fixture 18. The reheating can be conducted from the outside ofthe preform as shown in FIG. 1B, from the inside of the preform, or fromboth the outside and the inside of the preform, although heating justfrom the outside is the most common technique when forming PET bottles.

When the preform 10 reaches a desired temperature, the reheated preform10 is then ready for stretch blow molding. Referring next to FIGS. 1Cand 1D, the fixture 18 with the reheated preform 10 is held within amold cavity 30 having the contours to mold the polymer material into abottle. A gas, such as air or nitrogen, is injected into the internalvolume of the preform 10 through a nozzle in the fixture 18 as a pushrod 32 urges the polymer material to expand outwardly to conform to theinternal contours of the mold. Once the injection stretch blow moldinghas been completed, the finished bottle (not shown) is removed from themold. In one aspect, the present invention concerns injection stretchblow molding of polypropylene compositions using the equipment nowcommonly used by industry for injection stretch blow molding of PET.

Reheating agents added to the polypropylene composition to reducereheating time according to the invention include antimony (Sb),manganese (Mn), iron (Fe), carbon black, graphite, infra-red dyes,titanium (Ti), tungsten (W), and copper (Cu). Antimony and carbon blackare preferred reheating agents, and antimony is particularly preferred.The reheating agents preferably are added to the polypropylenecomposition in amounts from about 2 ppm to about 1000 ppm, morepreferably about 2 ppm to 350 ppm, and most preferably from about 2 ppmto 50 ppm, and with particle sizes in the range of about 10 nm to about100 microns, most preferably in the range 10 nm to 10 microns.

The reheating agents may be incorporated into the polypropylenecompositions in a number of ways. As one alternative, the reheatingagents may be directly mixed with the polypropylene granules prior tointroducing the mixture to an injection mold to form the preform. Asanother more preferred alternative, the reheat agents can be directlymixed with the polypropylene granules and passed through a twin screwcompounding extruder or similar piece of equipment to form a welldispersed and distributed polypropylene compound before being introducedto an injection moulding machine. As an alternate more preferredalternative, the reheating agents can be generated within thepolypropylene composition by in situ chemical reduction of a metalcompound with a reducing agent. As a fourth and even more preferredalternative, the reheating agents can be incorporated into thepolypropylene composition by one of the alternatives above, but in highconcentrations to form masterbatch granules. Then, such masterbatchgranules may be blended with polypropylene granules having a differentconcentration of reheating agent or no reheating agent or aconcentration of a different reheating agent to form a desiredpolypropylene composition containing one or more reheating agents indesired concentrations. For example, the polypropylene masterbatch mayincorporate reheating agent(s) in the form of particles having sizes inthe range of 10 nm to 100 microns, preferably less than 10 microns, andin an amount in the range of 50 ppm to 25,000 ppm, preferably less than1250 ppm.

The degree of reheating agent distribution and dispersion in thepolypropylene composition affects the reheating efficacy. That is tosay, the more evenly distributed and more widely dispersed the reheatingagents in the polymer, the better the reheating efficacy. Similarly,better distribution and dispersion of reheating agents in the polymerimproves the aesthetics of the polymer.

In one of the more preferred embodiments, the reheating agent is formedby an in situ chemical reduction of a metal compound with a reducingagent. The metal compound preferably contains one or more of antimony,titanium, copper, manganese, iron and tungsten, and the reducing agentpreferably is selected from the group consisting of one or more oforganic phosphorous acids, inorganic phosphorous acids, tannic, gallic,and pyrogallic acids, hydrazine, sulphites, tin II salts and nickelhydroxide, or any organic or inorganic compound with an electrochemicalpotential sufficient to reduce the metal compounds to the metallicstate.

Antimony as reheating agent preferably is formed by an in situ chemicalreduction of an antimony compound with a reducing agent, such ashypophosphorous acid or other organic phosphorous acid or inorganicphosphorous acid. Preferred antimony compounds include antimonytriglycolate (ATG), antimony triacetate (ATA) or antimony trioxide(ATO). The hypophosphorous acid reduces antimony compounds to antimony,which is dispersed in the polypropylene composition. The antimonyparticles appear to be more uniformly dispersed when introduced into thepolypropylene composition in this manner. In particular, we discoveredthat the antimony particles deposited by the reaction of ATA, ATO or ATGwith hypophosphorous acid and/or phosphorous acid have a particularlyfavorable particle size and are particularly well dispersed in thepolypropylene.

The present invention provides a method of making polypropylenecontainers, such as bottles, and other polypropylene articles that haveaesthetic characteristics close to polypropylene compositions withoutreheating agents. At the same time, the method also reduces the energyrequirements for reheating compared to polypropylene compositionswithout reheating agents. One immediate advantage is cost savings thoughreduced cycle time or energy savings. Yet another advantage of thepresent invention is that polypropylene can be used at a highertemperature than PET in subsequent processing, such as pasteurization orcleaning. In addition, polypropylene is recyclable to the same extent asPET.

The following examples further illustrate the present invention. Allparts and percentages are expressed by weight unless otherwisespecified.

EXAMPLES

Two methods for measuring the reheat temperature were used to assessreheating times for polypropylene compositions containing differentreheating agents. First, injection molded plaques were formed from thepolypropylene compositions and through heat times for such plaques weremeasured according to the procedure described below. Second, plaqueswere molded from the polypropylene compositions and surface reheat timesfor such plaques were measured according to the procedure describedbelow.

Preparing the Polypropylene Masterbatch Compositions

Polypropylene polymer granules with a size of about 2.5 to 4.0 mm inlength and 2.0 to 3.0 mm in diameter were mixed with reheating agentsaccording to the following steps. First, about 5-7 kg polypropylenepolymer was mixed with liquid paraffin (about 20 ml) in a plastic bag.The polymer in the bag was tumbled to coat the polymer granules with athin film of oil. Next, the reheating agents were added to the bag andthe admixture was tumbled again. Then, the coated polypropylene polymergranules were compounded using an APV MP2030 twin screw extruder,followed by a Boston Mathews single screw extruder fitted with a4-section cavity transfer mixer. The resultant compound was theninjection molded to form plaques for further testing.

Procedure for Measuring Plaque Through Heating Temperature

Polypropylene plaque through heating temperature measurements were madeas follows:

A polypropylene plaque of one hundred millimeters (100 mm) in diameterand four millimeters (4 mm) in cross-sectional thickness was used toevaluate the reheating time for various polypropylene compositions. Eachpolypropylene plaque of a polymer composition containing a reheatingagent and each plaque of the corresponding control composition were madeby a standard injection molding method using NB90 Negri Bossi 90teinjection molding equipment available from Negri Bossi of Milan Italy.All of the sample and control plaques were made to exactly the samedimension—100 mm in diameter and 4 mm in cross-sectional thickness. Theplaques were clean and free of surface contaminants, and had flat upperand lower surfaces.

As illustrated schematically in FIG. 2, a single IR radiation lamp 22 a,which is a 300 watt bulb made by Phillips, was positioned to heat oneside of the plaque 26. A series of flat thermocouples were positioned onthe opposite side of the plaque 26 to measure the radiated and conductedtemperature. The temperature measuring device was a TC-08 8 channelthermocouple data logger from Pico Technology Limited of The Mill House,Cambridge Street, St. Neots, PE19 1QB, and the temperature measuringsoftware was a Pico Technology Limited proprietary program supplied withthe TC-08 unit. The plaque was 165 mm below the bottom of the infra-redlamp. This test method reflects the amount of heat transferred throughthe plaque from the heated side. The temperature was recorded as themean of five measurements on five different plaques from each testsample and from a standard control plaque. We believe data from thisplaque through-heat method represents more realistically the heattransfer and distribution in an article, such as a preform, which isprocessed by a typical injection stretch blow molding method, or for anarticle which is processed by a reheating method followed by physicalforming, such as thermoforming.

The measuring apparatus was calibrated by using control plaques with adefined composition. These control plaques were tested repeatedly andshowed consistent reheat measurements. Then, the reheating times forplaques that were tested for comparison were expressed in reference tothe control. Two measurements were used: (1) time (seconds) to heat theplaque to 80° C.; and (2) temperature of the plaque after 300 secondsheating time. We have found that for proper comparison the plaques usedin this method should have the same starting ambient temperature. Newlymolded plaques should be cooled down to room temperature for asufficient amount of time before testing. In our tests, freshly moldedplaques were allowed to cool down at room temperature for at least 30minutes before the reheating test was conducted.

Referring to FIGS. 4, 5 and 6, the reheating profiles for plaques formedfrom a polypropylene control and from polypropylene compositionscontaining different amounts of antimony or carbon black were evaluated.

For the reheating test in FIG. 4, the compositions tested were: Sb1: 25ppm antimony formed by reducing reaction of equal parts by weight ofantimony triglycolate and hypophosphorous acid; Sb2: 25 ppm antimonymetal (milled) of particle size range 600 nm-2 microns; CB1: 25 ppm of40 nm carbon black; and CB2: 25 ppm of larger particle size 6-30 micronscarbon black. The reheating agents were all used in the same amount, 25ppm, based on the weight of the polypropylene. As shown in FIG. 4, thepolypropylene plaques that incorporated reheating agents all reheatedmore rapidly than the control polypropylene without reheating agents.All reached a desired 80° C. reheating temperature within 300 to 360seconds, whereas the control took more than 480 seconds to reach thisreheating temperature, which represents a 30% to 35% or better reductionin the reheating time.

FIGS. 5 and 6 compare the reheat time to 80° C. for the control andpolypropylene containing antimony or carbon black in varying amounts. InFIGS. 5 and 6, the reheating agents were: Sb1: 10 ppm as Sb formed usingequal parts by weight antimony triglycolate and hypophosphorous acid;Sb2: 25 ppm antimony metal (milled); CB1: 40 nm carbon black; and CB2:larger particle size 600 nm-2 microns carbon black. While both antimonyand carbon black can effectively reduce the reheating time (FIG. 5), thecarbon blacks at low levels can result in a significantly darkercoloration (L* significantly below 60 where the control L* was 75) (FIG.6), which can aesthetically detract from its use as a reheating agentfor polypropylene compositions. The combination of antimony triglycolateand hypophosphorous acid resulting in an antimony content in thepolypropylene of from 2 to 350 ppm, most preferably from 10 to 30 ppm,was particularly effective.

Introducing reheating agents in polypropylene compositions can result inundesired coloration of the final container, article or bottle product.The extent of coloration induced by reheating agents varies depending onthe type and the amount of agents used. For a given reheating agent, theless amount of the agent is used, the less coloration will be in thefinal product. If the amount of the reheating agent used is low enoughto minimize the unwanted coloration to an accepted level, that amountmay not be sufficient to reduce reheating time. The challenge,therefore, is to find a reheating agent that can effectively reduce thereheating time and still produce a final container, article or bottlewith minimum coloration.

We used a color measurement (L*) from the Gardner calorimeter toevaluate coloration of polypropylene compositions caused by differentreheating agents. The color measurement (L*) reflects the absorption andscattering of light by the testing material. The Gardner BYK Color-Viewspectrophotometer Model No. 9000 is available from BYK Gardner, Inc.,Columbia, Md. USA. L* values for polypropylene plaques made frompolypropylene compositions containing different reheating agents indifferent amounts were measured.

The L* measurements shown in FIG. 6 illustrate the relationship betweenthe coloration of polypropylene plaques and the amount of reheatingagents in the polypropylene compositions from which the plaques weremade. The control plaque was polypropylene without adding reheatingagents. The control plaque had an L* value of about 75. The lessercoloration a polypropylene composition has, the closer its L* value isto that of the control. As shown in FIG. 6, the polypropylenecomposition with 10 ppm antimony (composition Sb1) had the aestheticcharacteristics that are the closest to those of the polypropylenewithout any reheating agents (control). In these examples, the sampleshaving L* values of about 60 and above as compared to L* of about 75 forthe polypropylene control had color aesthetics that were at least 80% ofthe L* value of the control.

Table 1 below summarizes the energy savings due to the reduction ofreheating time and the relative color aesthetics of these certain testedpolypropylene compositions. TABLE 1 Energy Savings and Aesthetics ofReheating Agents in Blowing Trials. Blowing Trial 1 Reheating EnergyAesthetics (L*) Agent Level (ppm) Saving (%) (Granules) Control 0 075.71 Sb1 10 10 68.31 CB1 10 20 51.58 Sb1: antimony (as describedabove); CB1: 40 nm carbon black (as described above). Blowing Trial 2Reheating Cycle time Aesthetics (L*) Agent Level (ppm) Saving (%)(Granules) Control 0 0 73.94 Sb1 10 24 65.26 Sb2 10 24 63.79 CB1 5 2459.96 CB1 10 31 47.74 Sb1: antimony (as described above); Sb2; antimony(as described above) CB1: 40 nm carbon black (as described above).Blowing Trial Details

The preforms for the blowing trial were made on a laboratory scalecombined injection moulding and blowing machine. The preforms wereinjection moulded with typical polypropylene processing conditions—themelt temperature was 220° C., the mould temperature was 15° C., and thecycle time was 27 seconds. We then used a separate laboratory blowingmachine which had been designed specifically for polypropylene. Thisblowing machine had two heating ovens, each having 10,000 watts heatingcapacity and each fitted with forced air ventilation. Several spinningpreform holders were used to index the preforms through two ovensseparated by an air gap. Each preform was indexed through the first ovenin about 60 to 80 seconds. Then, the preform was indexed through an airspace to allow the heated preform to equilibrate for about 60 to 80seconds. Next, the preform was indexed through a second oven for about60 to 80 seconds, and then, after a further 10-second period indexing inair, it was delivered to the blowing station.

For the energy saving experiment, the machine indexing speed was set ata constant 750 bottles per hour output, and the oven settings wereadjusted (reduced) for each sample to provide an optimum injectionmoulded bottle. Energy savings were then calculated based on the lesseramount of heating energy required to reheat the bottles formed from apolypropylene composition that contained reheating agents versus theamount of heating energy required to reheat the control bottle. In Table1, energy savings are expressed as a percentage.

For the cycle time saving experiment, both ovens were set to a constant8900 watts output (for a combined 17,800 watts output). The cycle timewas then increased until the preforms were heated sufficiently to allowthe optimum blowing of the bottle. The increased cycle time of eachsample was calculated against the control cycle time and expressed as apercentage.

The results in Table 1 indicate that the use of 10 ppm Sb produced fromantimony triglycolate with hypophosphorous acid reduced the preformreheating energy and cycle time to the same level as 5 ppm carbon black,but the color and visual aesthetics of the polypropylene granulecontaining antimony was much better than the granules that containedcarbon black. Thus, the resulting bottles formed from the polypropylenecomposition with antimony had better color and visual aesthetics thandid the bottles formed from the polypropylene composition containing 5ppm or 10 ppm carbon black.

The preforms with reheating agents as compared to the control preformsproduced energy savings in the range of about 10% to 30%, preferably 15% to 20%, and cycle time savings in the range of about 25% to 35% (seeTable 1). Thus, the polypropylene compositions with reheating agentsshorten reheating time and still produce bottles with acceptable colorand visual aesthetics.

We further explored the reheating and aesthetic characteristics ofdifferent types of antimony reagents. More specifically, we testedantimony triglycolate, antimony trioxide and antimony triacetate atdifferent concentrations. We additionally explored the reheating andaesthetic characteristics of other reheating agents. We also testeddifferent methods for mixing the reheating agents with thepolypropylene, namely, by direct application to the polypropylenegranules, and by mixing polypropylene granules with masterbatch granulescontaining high concentrations of reheating agents. For reducing agents,different phosphorous acids with different dosages were also tested. SeeTables 2 and 3 below for a summary of these results.

We found that adding antimony as a reheating agent was preferred tocarbon black in view of the aesthetics. We further found that addingantimony compounds in combination with phosphoric acid, preferablyhypophosphoric acid, more thoroughly disperses the metal particles inthe polypropylene composition. Lastly, the antimony triglycolate,antimony trioxide or antimony triacetate with hypophosphoric acidcombinations that resulted in from 2 ppm to 350 ppm antimony metalparticles in the polypropylene composition achieved the best results forreheat performance and color aesthetics.

Procedure for Measuring Plaque Surface Reheat Temperature

Plaques injection molded from polypropylene compositions as specified inTables 1, 2 and 3 were rotated while heated by radiation emitted by anIR heating lamp (a 175 Watt lamp model IR-175C-PAR from Phillips at2400°K) (See, e.g., FIG. 2A). An infra-red pyrometer (model numberCyclops 300AF from Minolta Land) (not shown in FIG. 2) was positioned onthe opposite side of the plaque with respect to the IR lamp. The surfacetemperature of the plaque was monitored and recorded during reheating.

The plaques tested contained various reheating agents, such as antimony(Sb), manganese (Mn), iron (Fe), titanium (Ti), tungsten (W), copper(Cu), graphite, infra-red dye and carbon black, in various amounts. Theresults are reported in Tables 2 and 3 below. For the compositions setforth in Tables 2 and 3, the controls were plaques made frompolypropylene without reheating additives. All of the reheating agentstested improved the plaque surface reheating time as compared with thecontrol polypropylene. However, certain of the reheating agents whenincorporated into polypropylene granules (and plaques) had L* valuesmuch lower than the target L* (80% of the control), and thus may not beas suitable for use in injection stretch blow molding of bottles. Thesecompositions still may have utility when making other polypropylenearticles with thermoforming techniques.

Data for surface reheating times for certain of the polypropylenecompositions that included reheating agent, compared to the control C1,is shown graphically in FIG. 3. TABLE 2 Comparison of L* and ReheatingCharacteristics Through- Through- Sample Control Plaque heat heat in 300secs No. Target Formulation Polymer Granule L* L* to 80° C. (sec) (° C.)C1 Eltex PPC KV 276 73.99 75.17 500 69.2 Control A C2 Eltex PPC KV 27670.25 75.11 458 69.2 Control A C3 Eltex PPC KV 276 71.24 75.71 419 70.4Control A C4 Borealis PP RE420M0 70.96 78.84 466 67.3 Control B C5Borealis PP RE420M0 68.73 79.4 401 72.1 Control B  1 5 ppm Cu A 66.1175.28 439 67  2 50 ppm Cu A 53.90 61.18 386 72  3 5 ppm coarse carbon A56.73 75.85 366 74.3 black  4 10 ppm coarse carbon A 48.5 72.64 334 76.7black  5 20 ppm coarse carbon A 37.83 37.30 331 76.7 black  6 10 ppmElftex 254 CB B 46.85 53.73 325 77.2  7 2 ppm 20 nm fine CB N B 61.5173.11 443 69.1  8 5 ppm 20 nm fine CB N B 53.96 61.32 442 69.2  9 10 ppmfine carbon B 45.82 53.93 336 76.4 black 10 25 ppm fine carbon B 39.1031.95 336 76.7 black 11 50 ppm fine carbon B 26.32 15.53 292 80.4 black12 Infra-red dye-10 ppm B 63.54 75.07 407 71.6 Avecia pro-jet 830 NP 1310 ppm Fe A 64.80 72.88 412 71 14 100 ppm 100 nm Fe N A 47.48 49.49 34175.4 15 200 ppm M Fe N A 70.21 78.94 498 66.9 16 10 ppm Fe(III)oxide B58.36 71.54 413 71.6 17 10 ppm graphite EP B 64.15 72.16 389 72.9 102018 10 ppm 1-2micron B 63.89 72.94 366 73.8 graphite 19 200 ppm M Mn N A62.44 71.00 417 70.6 20 2.5 ppm Sb via A 67.22 76.15 436 70 ATG/H₃PO₂ 215 ppm Sb via ATG/ A 59.84 67.64 418 70.4 H₃PO₂₊ 10 ppm Ti 22 5 ppm fineSb B 68.00 75.65 405 71.6 23 10 ppm Sb via ATG/ A 59.91 69.20 388 72.7H₃PO₂ 24 10 ppm fine Sb B 63.37 72.32 380 73.6 25 20 ppm Sb via ATG/ B57.74 69.44 349 75 H₃PO₂ 26 30 ppm Sb via ATG/ B 54.27 65.01 337 76.2H₃PO₄ 27 50 ppm Sb via ATG/ A 64.48 71.17 327 77.3 H₃PO₃ N 28 50 ppm Sbvia A 73.62 78.27 457 68.8 ATG/TNPP low N 29 50 ppm Sb via A 70.19 77.89457 68.8 ATG/TNPP/H₃PO₂ 30 50 ppm fine Sb B 46.58 51.69 315 78.8 31 100pm Sb/P A 58.39 68.8 390 72.2 32 50 ppm Sb via ATA/ A 56.30 65.67 39472.2 H₃PO₂ N 33 50 ppm Sb via H₃PO₃ N A 58.86 69.92 415 70.6 34 5 ppmcoarse Sb A 66.39 75.85 391 72.7 35 10 ppm coarse Sb A 64.3 72.64 36574.4 36 25 ppm coarse Sb A 57.31 64.25 329 77.2 37 100 ppm coarse Sb A42.19 39.68 287 81.8 38 200 ppm M Sb N A 40.43 39.29 301 79.6 39 10 ppmSb via Sb₂0₃/ B 66.3 76.81 417 71 H₃PO₂ 40 200 ppm Ti N A 29.35 20.87352 74.5 41 100 ppm 100 nm W N A 58.98 68.73 415 70.8M/b: masterbatch.N: nucleated.M: milled.ATG: antimony triglycolate.ATA: antimony triacetate.TNPP: trisnonylphenyl phosphite.H₃PO₃ = phosphorous acid,H₂PO₃ = Hypophosphorous acid

TABLE 3 Comparison of L* and Reheating Characteristics for MasterbatchCompositions Through- M/b M/b let Final Through- heat in Sample M/bTarget Control granule down granule heat to 300 secs No. FormulationPolymer L* ratio L* 80° C. (sec) (° C.) 42 250 ppm fine 20 nm B 17.5712.5:1 47.74 — — CB 43 250 ppm copper (I) B 61.21 — 61.21 412 71.1 oxide44 250 ppm copper (II) B 60.32 — 60.32 427 70.6 oxide 45 125 ppm AveciaB 39.7 12.5:1 63.54 — — projet 803NP (dye) 46 1000 ppm Fe A 23.09 —23.09 301 79.8 47 250 ppm Fe(II) B 62.67 — 62.67 398 71.9 oxalate +H₃PO₂ 48 250 ppm Fe(III) B 66.5 — 66.5 415 70.5 oxalate + H₃PO₂ 49 250ppm Fe(II + III) B 52.15 — 52.15 327 77.7 oxide 50 250 ppm Aldrich 1-2 B30.87 25.0:1 63.89 — — micron graphite 51 1000 ppm M Mn A 47.47 — 47.47— — 52 250 ppm ATG + H₃PO₂ B 28.83 25.0:1 63.37 — — 53 250 ppm Aldrich B65.92 — 65.92 374 73.5 fine Sb₂O₃ 54 250 ppm Aldrich B 37.25 25.0:1 66.3— — fine Sb₂O₃ + H₃PO₂ 55 1000 ppm Sb/P A 32.97 — 32.97 421 70.4 56 266ppm milled Sb B 34.19 25.0:1 63.79 — — 57 1000 ppm 100 nm A 24.06 —24.06 329 773   Ti 58 1000 ppm 100 nm W A 31.45 — 31.45 — —

Polypropylene compositions with reheating agents according to theinvention preferably result in at least a 10% reduction in reheatingtime, more preferably at least a 25% reduction in reheating time, andmost preferably at least a 35% reduction in reheating time, as comparedto the reheating time for the control composition. Thus, from Tables 2and 3, the polypropylene compositions with reheating agents that hadreheat times to 80° C. of 375 seconds and under, and particularly 300seconds and under were most advantageous.

The invention has been illustrated by detailed description and examplesof the preferred embodiment. Various changes in form and detail will bewithin the skill of persons skilled in the art. Therefore, the inventionmust be measured by the claims and not by the description of theexamples or the preferred embodiments.

1. A method for injection stretch molding a polypropylene bottle,comprising: (a) forming a preform from a polypropylene compositioncontaining a reheating agent, wherein the reheating agent comprises oneor more metal particles, and wherein said metal particles are selectedfrom the group consisting of one or more of antimony, titanium, copper,manganese, iron and tungsten; (b) reheating the preform to a desiredtemperature, wherein the time for the preform to reach the desiredtemperature is less than the time for reheating to the desiredtemperature a control preform of equivalent dimensions that is formedfrom the polypropylene composition without the reheating agent; and (c)injection stretch blow molding the reheated preform to form the bottle;wherein the polypropylene composition containing a reheating agent whenit is in a granule form prior to forming the preform has an L* value asmeasured by the Gardner color test of at least about 80% of an L* valueof the polypropylene composition in granule form in the absence of thereheating agent.
 2. (canceled)
 3. A method for injection stretch moldinga polypropylene bottle, comprising: (a) forming a preform from apolypropylene composition containing a reheating agent, wherein thereheating agent comprises carbon black, graphite or an infra-red dye inthe form of particles incorporated into the polypropylene composition inan amount in the range of 2 ppm to 1000 ppm; (b) reheating the preformto a desired temperature, wherein the time for the preform to reach thedesired temperature is less than the time for reheating to the desiredtemperature a control preform of equivalent dimensions that is formedfrom the polypropylene composition without the reheating agent; and (c)injection stretch blow molding the reheated preform to form the bottle;wherein the polypropylene composition containing a reheating agent whenit is in a granule form prior to forming the preform has an L* value asmeasured by the Gardner color test of at least about 80% of an L* valueof the polypropylene composition in granule form in the absence of thereheating agent.
 4. The method of claim 1, wherein the reheating agentis incorporated into the polypropylene composition in the form ofparticles having sizes in the range of 10 nm to 100 microns and in anamount in the range of 2 ppm to 1000 ppm.
 5. The method of claim 1 or 3,wherein the reheating agent is incorporated into the polypropylenecomposition in the form of particles having sizes in the range of 10 nmto 10 microns and in an amount in the range of 2 ppm to 50 ppm.
 6. Themethod of claim 1, wherein the reheating agent is generated within thepolypropylene composition by in situ chemical reduction of a metalcompound with a reducing agent.
 7. The method of claim 6, wherein themetal compound contains one or more of antimony, titanium, copper,manganese, iron and tungsten, and the reducing agent is selected fromthe group consisting of one or more of organic phosphorous acids,inorganic phosphorous acids, tannic, gallic, and pyrogallic acids,hydrazine, sulphites, tin II salts and nickel hydroxide.
 8. The methodof claim 6, wherein the metal compound is antimony triglycolate and thereducing agent is hypophosphorous acid.
 9. The method of claim 1,wherein the reheating agent is incorporated into the polypropylenecomposition in the form of particles having sizes in the range of 10 nmto 100 microns and in an amount in the range of 50 ppm to 25,000 ppm toform a polypropylene masterbatch.
 10. A polypropylene bottle madeaccording to the method of claim 1 or
 3. 11.-33. (canceled)
 34. Themethod of claim 3, wherein the reheating agent is incorporated into thepolypropylene composition in the form of particles having sizes in therange of 10 nm to 100 microns.
 35. The method of claim 3, wherein thereheating agent is incorporated into the polypropylene composition in anamount in the range of from 2 ppm to 350 ppm.
 36. The method of claim 3,wherein the reheating agent is incorporated into the polypropylenecomposition in an amount in the range of from 2 ppm to 50 ppm.
 37. Themethod of claim 6, wherein the metal compound is antimony trioxide. 38.The method of claim 6, wherein the metal compound is antimony trioxideand the reducing agent is hypophosphorous acid.
 39. A method forinjection stretch molding a polypropylene bottle, comprising: (d)forming a preform from a polypropylene composition containing areheating agent selected from metal particles that intrinsically absorbradiation in the wavelength region of 500 nm to 2000 nm; (e) reheatingthe preform to a desired temperature, wherein the time for the preformto reach the desired temperature is less than the time for reheating tothe desired temperature a control preform of equivalent dimensions thatis formed from the polypropylene composition without the reheatingagent; and (f) injection stretch blow molding the reheated preform toform the bottle; wherein the polypropylene composition containing areheating agent when it is in a granule form prior to forming thepreform has an L* value as measured by the Gardner color test of atleast about 80% of an L* value of the polypropylene composition ingranule form in the absence of the reheating agent.
 40. The method ofclaim 39, wherein the reheating agent is incorporated into thepolypropylene composition in the form of particles having sizes in therange of 10 nm to 100 microns and in an amount in the range of 2 ppm to1000 ppm.
 41. The method of claim 39, wherein the reheating agent isincorporated into the polypropylene composition in the form of particleshaving sizes in the range of 10 nm to 10 microns and in an amount in therange of 2 ppm to 50 ppm.
 42. The method of claim 39, wherein thereheating agent is generated within the polypropylene composition by insitu chemical reduction of a metal compound with a reducing agent. 43.The method of claim 39, wherein the reheating agent is incorporated intothe polypropylene composition in the form of particles having sizes inthe range of 10 nm to 100 microns and in an amount in the range of 50ppm to 25,000 ppm to form a polypropylene masterbatch.